Method and apparatus for decomposing nitrogen oxide

ABSTRACT

A method for decomposing nitrogen oxide includes: contacting a gas stream comprising nitrogen oxide with a device, the device comprising: a first electrode, an opposite second electrode, an electrolyte between the first and the second electrodes, and a power supply; and applying in a pulse mode an electrical current from the power supply to the first and the second electrodes to decompose nitrogen oxide. An associated apparatus is also described.

BACKGROUND

Embodiments of the present invention relate generally to methods andapparatuses for decomposing nitrogen oxide.

Nitrogen oxide (NOx, including NO and/or NO2) is undesirable for theenvironment and has to be controlled. Some approaches have been proposedto decompose nitrogen oxide into nitrogen and oxygen. However, someapproaches use hazardous compound such as ammonia, and/or causesecondary pollution by producing ammonium sulfate, besides being complexand expensive. Other approaches consume a relatively large amount ofpower while applying electricity in decomposing nitrogen oxide.

Therefore, while some of the proposed approaches have general use invarious industries, it is desirable to provide new methods andapparatuses for decomposing nitrogen oxide.

BRIEF DESCRIPTION

In one aspect, the invention relates to a method for decomposingnitrogen oxide, comprising: contacting a gas stream comprising nitrogenoxide with a device, the device comprising: a first electrode, anopposite second electrode, an electrolyte between the first and thesecond electrodes, and a power supply; and applying in a pulse mode anelectrical current from the power supply to the first and the secondelectrodes to decompose nitrogen oxide.

In another aspect, the invention relates to an apparatus for decomposingnitrogen oxide, comprising: a gas source for providing a gas streamcomprising nitrogen oxide; and a device in fluid communication with thegas source and comprising: a first electrode, an opposite secondelectrode, an electrolyte between the first and the second electrodes,and a power supply comprising a controller for applying in a pulse modean electrical current from the power supply to the first and the secondelectrodes to decompose nitrogen oxide.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings,wherein:

FIG. 1 illustrates a schematic cross sectional view of an apparatus of afirst embodiment of the invention;

FIG. 2 illustrates a schematic cross sectional view of an apparatus of asecond embodiment of the invention;

FIG. 3 illustrates a schematic cross sectional view of an apparatus of athird embodiment of the invention;

FIG. 4 illustrates a schematic cross sectional view of an apparatus of afourth embodiment of the invention;

FIG. 5 shows the NO conversion percentage of a gas stream (80 ml/min,400 ppm NO balanced with He) in the reactor using aLa0.6Sr0.4Ni0.3Mn0.703—Zr0.89Sc0.1Ce0.01O2-x layer as the cathode at600° C. as a function of time applied with and stopped from 50 mA ofdirect current; and

FIG. 6 illustrates the NO conversion percentage of a gas stream (80ml/min, 400 ppm NO balanced with He) at 600° C. in reactors using aNiO—Zr0.89Sc0.1Ce0.01O2-x layer and aLa0.6Sr0.4Ni0.3Mn0.7O3—Zr0.89Sc0.1Ce0.01O2-x layer as cathodes beforeapplying and 5 hours after stopping 50 mA of electrical current,respectively.

DETAILED DESCRIPTION

Unless defined otherwise, technical and scientific terms used hereinhave the same meaning as is commonly understood by one of ordinary skillin the art to which this disclosure belongs. The terms “first”,“second”, and the like, as used herein do not denote any order,quantity, or importance, but rather are used to distinguish one elementfrom another. The use of “including”, “comprising” or “having” andvariations thereof herein are meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about” is not to be limited to the precise valuespecified. In some instances, the approximating language may correspondto the precision of an instrument for measuring the value. Here andthroughout the specification and claims, range limitations may becombined and/or interchanged; such ranges are identified and include allthe sub-ranges contained therein unless context or language indicatesotherwise.

In the specification and the claims, the singular forms “a”, “an” and“the” include plural referents unless the context clearly dictatesotherwise. Moreover, the suffix “(s)” as used herein is usually intendedto include both the singular and the plural of the term that itmodifies, thereby including one or more of that term.

As used herein, the term “or” is not meant to be exclusive and refers toat least one of the referenced components (for example, a material)being present and includes instances in which a combination of thereferenced components may be present, unless the context clearlydictates otherwise.

Reference throughout the specification to “some embodiments”, and soforth, means that a particular element (e.g., feature, structure, and/orcharacteristic) described in connection with the invention is includedin at least one embodiment described herein, and may or may not bepresent in other embodiments. In addition, it is to be understood thatthe described inventive features may be combined in any suitable mannerin the various embodiments.

Embodiments of the present invention relate to methods and apparatusesfor decomposing nitrogen oxide.

As used herein the term “nitrogen oxide” or the like refers to a gascomprising molecules including both oxygen and nitrogen, for example,nitrogen monoxide, nitrogen dioxide, or a combination thereof.

Please refer to FIGS. 1, 2, 3 and 4, an apparatus 10, 20, 30, 40 ofembodiments of the invention includes a gas source 11, 21, 31, 41 forproviding a gas stream 12, 22, 32, 42 comprising nitrogen oxide and adevice 100, 200, 300, 400 in fluid communication with the gas source 11,21, 31, 41.

The gas stream comprising nitrogen oxide may be from a variety of gassources. In some embodiments, the gas sources are exhaust gas sourcesfrom gas turbines, internal combustion engines, or combustion devices.In some embodiments, the gas source comprises a conduit, a channel, or atube for the passage of the gas stream.

In some embodiments, the device 100, 200, 300, 400 includes a firstelectrode 101, 201, 301, 401, an opposite second electrode 102, 202,302, 402, an electrolyte 103, 203, 303, 403 between the first and thesecond electrodes, and a power supply 104, 204, 304, 404 having acontroller 114, 214, 314, 414 for applying in a pulse mode an electricalcurrent from the power supply 104, 204, 304, 404 to the first and thesecond electrodes to decompose nitrogen oxide.

In some embodiments, nitrogen oxide can be directly decomposed in thedevice 100, 200, 300, 400 before an electrical current is applied. Whena gas stream comprising nitrogen oxide is contacted with the device,nitrogen oxide is decomposed in the cathode in a reaction such as:NO=½N2+½O2.

However, as can be seen from examples described hereafter, when theelectrical current is applied, besides the direct decomposition of NOdescribed above, nitrogen oxide can also be decomposed in the cathode inan electrochemical reaction of NO+2e→½N2+O2—. The oxygen ions producedthereby travel from the cathode through the electrolyte into the anodeto be oxidized into oxygen in a reaction of O2-2e→½O2. A total reactionin the device is: NO=½N2+½O2. The decomposition rate of nitrogen oxideincreases and after the electrical current is stopped, the conversion(decomposition) rate of nitrogen oxide is still higher for a long timeperiod than the conversion rate of nitrogen oxide before applying theelectrical current.

Therefore, by applying an electrical current in a pulse mode, thedecomposition of nitrogen oxide can be achieved at a higher conversionrate than without applying an electrical current and with less powerconsumption than continuously applying an electrical current.

The decomposition of nitrogen oxide may be at any suitable temperature.In some embodiments, the step of applying the electrical current is at atemperature in a range from about 300° C. to about 1000° C.

As used herein, the term “pulse mode” refers to intermittently applyingand removing electric current, in contrast to a continuous applicationof current during service. The manner and duration of respectivelyapplying and removing the electrical current in the pulse mode may bedependent upon the specific apparatus, the specific gas stream, and thedecomposition environment, as long as the conversion rate of nitrogenoxide and the power consumption are satisfactory in the specificcircumstance.

In some embodiments, in the pulse mode the electrical current is appliedand removed alternately. In some embodiments, in the pulse mode theelectrical current is applied for a time period different from a timeperiod when the electrical current is stopped. In some embodiments, inthe pulse mode the electrical current is applied for a time period thesame as a time period when the electrical current is stopped.

The electrical current may be any electrical current that can be used todecompose nitrogen oxide at a conversion rate higher than that of beforean electrical current is applied. In some embodiments, the electricalcurrent is direct current. In some embodiments, the electrical currentis applied by jumping to the designed value directly. In someembodiments, the electrical current is applied by sweeping to thedesigned value slowly.

The controller 114, 214, 314, 414 may be any mechanism that controls theon and off and/or increasing and decreasing of the electrical current.In some embodiments, the controller is a switch for turning on and offthe electrical current.

In some embodiments, the first electrode 101, 201, 301, 401 is an anode.The anode may include any material that oxidizes oxygen ions to oxygen,and any other materials that can be used in the anode. In someembodiments, the anode comprises a manganite, such as lanthanumstrontium manganite (LSM), a non-limiting exemplary composition of whichincludes (La0.8Sr0.2)0.95MnO3; a combination of platinum and yttriastabilized zirconia; a combination of platinum and gadolinium-dopedceria; or any combination thereof.

In some embodiments, the second electrode 102, 202, 302, 402 is acathode. The cathode may include any material that decomposes nitrogenoxide to nitrogen and oxygen ions, and any other materials that can beused in the cathode.

In some embodiments, the cathode includes catalysts catalyzing thedecomposition of nitrogen oxide. In some embodiments, the cathodecomprises catalysts catalyzing the decomposition of nitrogen oxide withlittle or no impact by the presence of oxygen. The oxygen coexistingwith nitrogen oxide may be discharged from the cathode.

In some embodiments, the cathode has adsorption materials that adsorbnitrogen oxide. Examples of the adsorption material include, but are notlimited to, magnesium oxide, calcium oxide, sodium oxide, potassiumoxide, barium oxide, and strontium oxide.

In some embodiments, the cathode comprises a manganite, such aslanthanum strontium nickel manganite (LSNM), an exemplary composition ofwhich includes, but is not limited to, La0.6Sr0.4Ni0.3Mn0.7O3; nickeloxide (NiO); a combination of LSNM and gadolinium doped ceria (GDC,e.g., Gd0.1Ce0.9O1.95); a combination of LSNM and scandia stabilizedzirconia (SSZ, e.g., Zr0.89Sc0.1Ce0.01O2-x) (such as in a 50 wt %ratio); a combination of LSNM, NiO and SSZ (such as, a ratio of 40 wt %,30 wt %, and 30 wt %); a combination of NiO and SSZ (such as in a 50 wt% ratio); a combination of platinum with yttria-stabilized zirconia; acombination of platinum with GDC; or any combination thereof.

In some embodiments, as is shown in FIGS. 3 and 4, the device 30, 40comprises an adsorption layer 305, 405 disposed over the secondelectrode 302, 402, either directly, or with one or more intermediatelayers therebetween. The adsorption layer may comprise any adsorptionmaterial that adsorbs nitrogen oxide, such as those describedpreviously. The adsorption material may be distributed inside thecathode without forming an extra layer.

In some embodiments, the apparatus comprises a current collector (notshown). The current collector may be made of any electrically conductivematerials such as metals or metal alloys and be in any forms suitablefor use in supplying or withdrawing electrical current from theelectrodes. In some embodiments, the current collector is made ofnickel. In some embodiments, the current collector is in the form ofmesh, porous film, foam, or any combination thereof. In someembodiments, the current collector is nickel foam. In some embodiments,a porosity of a porous metallic current collector is in a range fromabout 25% to about 99%.

In some embodiments, the current collector is a mechanical support forthe first and the second electrodes.

In some embodiments, the device comprises a current collector disposedover the second electrode, either directly, or with one or moreintermediate layers therebetween.

The electrolyte may include any material that has a suitable level ofoxygen ion conductivity and any other suitable material. In someembodiments, the electrolyte comprises GDC, such as Gd0.1Ce0.9O1.95;SSZ, such as Zr0.89Sc0.1Ce0.01O2-x; oxide materials from thebarium-zirconium-cerium-yttrium (BZCY) family, such asBaZr0.7Ce0.2Y0.1O3; or any combination thereof. In some embodiments, theelectrolyte includes bismuth oxide, zeolite, alumina, silica, aluminumnitride, SiC, nickel oxide, iron oxide, copper oxide, calcium oxide,magnesium oxide, zinc oxide, aluminum, yttria stabilized zirconia,scandia stabilized zirconia, perovskite oxides, lanthanum strontiumcalcium manganese, lanthanum silicate, Nd9.33(SiO4)6O2, AlPO4, B2O3, andR2O (R stands for an alkaline metal), AlPO4—B2O3—R2O glass which carriesout the main component of Na and the K, porous SiO2—P2O5 system glass, Yaddition BaZrO3, Y addition SrZrO3 and Y addition SrTiO3, strontiumdoping lanthanum manganite, a lanthanum strontium cobalt iron oxide(La—Sr—Co—Fe system perovskite type oxide), a La—Sr—Mn—Fe systemperovskite type oxide, a Ba—Sr—Mn—Fe system perovskite type oxide, orany combination thereof.

A dense electrolyte is, in an embodiment, used for mitigating the mixingof the gases of the cathode and the anode and reducing the ohmicresistance of the electrolyte. Low ohmic resistance is in an embodimentpreferred for energy saving in the NOx reduction process.

Each of the electrode, the electrolyte, the current collector, and theadsorption layer may be a single layer or comprise more than one layerdepending on the needed flexibility, gas diffusion capability, andporosity. Multiple layers may be the same as or different from eachother and connected in suitable ways. In each single layer, thecomposition may be the same or different through at least one dimensionthereof.

The device may be of any configuration suitable for decomposing nitrogenoxide. In some embodiments, as is shown in FIGS. 1 and 3, the device100, 300 is of a planar configuration. In some embodiments, as is shownin FIGS. 2 and 4, the device 200, 400 is of a tubular configuration andcomprises a space 206, 406 therein.

The device described herein may be prepared by providing a currentcollector and applying sequentially different layers on both sidesthereof, or providing any of other layers and laminating differentlayers on either/both sides thereof. The layers may beformed/applied/laminated by any suitable means such as extruding, dipcoating, spraying and printing.

EXAMPLES

The following examples are included to provide additional guidance tothose of ordinary skill in the art in practicing the claimed invention.These examples do not limit the invention as defined in the appendedclaims.

Example 1 La0.6Sr0.4Ni0.3Mn0.7O3 Synthesis

La2O3, SrCO3, Mn(AC)2.4H2O and NiO were ball milled in EtOH and calcinedat 1300° C. for 8 hours to prepare La0.6Sr0.4Ni0.3Mn0.7O3. X-raydiffraction (XRD) analyses confirmed that a pure phase ofLa0.6Sr0.4Ni0.3Mn0.7O3 was obtained.

Example 2 Reactor Preparation

Two 7.5 cm long one-end open (La0.8Sr0.2)0.95MnO3 tubes were fabricatedby extruding. The outer diameter of each tube was about 1 cm, and theinner diameter was about 0.7 cm.

A dense Zr0.89Sc0.1Ce0.01O2-x electrolyte film was coated on each(La0.8Sr0.2)0.95MnO3 tube and was co-sintered with the(La0.8Sr0.2)0.95MnO3 tube at 1250° C.

A layer of La0.6Sr0.4Ni0.3Mn0.7O3 and Zr0.89Sc0.1Ce0.01O2-x(La0.6Sr0.4Ni0.3Mn0.7O3—Zr0.89Sc0.1Ce0.01O2-x layer, 50 wt % ratio) anda layer of NiO and Zr0.89Sc0.1Ce0.01O2-x (NiO—Zr0.89Sc0.1Ce0.01O2-xlayer, 50 wt % ratio) were respectively deposited on theZr0.89Sc0.1Ce0.01O2-x electrolyte films and sintered at around 900-1100°C. to obtain two reactors. The active area of each ofLa0.6Sr0.4Ni0.3Mn0.7O3—Zr0.89Sc0.1Ce0.01O2-x andNiO—Zr0.89Sc0.1Ce0.01O2-x layers was about 10 cm2.

A layer of porous platinum paste was applied to each ofLa0.6Sr0.4Ni0.3Mn0.7O3—Zr0.89Sc0.1Ce0.01O2-x andNiO—Zr0.89Sc0.1Ce0.01O2-x layers to form a porous metallic currentcollector.

The microstructures of the reactors were analyzed. As a typical example,SEM images of the cross section of the(La0.8Sr0.2)0.95MnO3/Zr0.89Sc0.1Ce0.01O2-x/NiO—Zr0.89Sc0.1Ce0.01O2-xreactor show that the (La0.8Sr0.2)0.95MnO3 layer had a porous structurewith a low porosity, the Zr0.89Sc0.1Ce0.01O2-x layer had a densestructure, while the NiO—Zr0.89Sc0.1Ce0.01O2-x layer had a porousstructure with a high porosity.

Example 3 Decomposition of Nitrogen Oxide

The reactors were each put inside an alumina tube. The inner diameter ofthe alumina tube was about 2 cm. A gas stream (400 ppm NO balanced withHe, 80 ml/min) was fed into the alumina tube passing through the outersurface of the reactor at a temperature of 600° C. Direct current (DC)of 50 mA was applied on each reactor for about 900 minutes before beingstopped.

The La0.6Sr0.4Ni0.3Mn0.7O3—Zr0.89Sc0.1Ce0.01O2-x layer and theNiO—Zr0.89Sc0.1Ce0.01O2-x layer were assigned as cathodes, where thedirect decomposition of NO and electrochemical NO reduction took place.The (La0.8Sr0.2)0.95MnO3 layer was the anode, where the oxidation ofoxygen ions took place. The corresponding voltage between anode andcathode was in the range of 1-1.5 V. Gas chromatography equipped with aPQ column and a RAE7800 gas sensor were used to detect NO and NO2 withan accuracy of 1 ppm and 0.1 ppm, respectively.

FIG. 5 shows the NO conversion percentage of the reactor using theLa0.6Sr0.4Ni0.3Mn0.7O3—Zr0.89Sc0.1Ce0.01O2-x layer as the cathode layerat 600° C. increased gradually from about 5% to about 40% in about 900minutes after applying the direct current of 50 mA. The 5% NOxconversion rate before applying the DC is the direct catalytic NOxdecomposition activity of the reactor. After the electrical current wasstopped, the NO conversion rate gradually decreased to about 20% after300 minutes, which is much higher than the initial 5% conversion ratebefore applying the electrical current. This suggests that the DCactivated the reactor for the NOx decomposition. Therefore, thisexperiment demonstrates that nitrogen oxide may be decomposed at ahigher conversion rate by applying and stopping the electrical currentalternately in a pulse mode than without applying an electrical current.

NO conversion rates before applying and about 5 hours after stopping 50mA of direct current in the reactors using the NiO—Zr0.89Sc0.1Ce0.01O2-xlayer and the La0.6Sr0.4Ni0.3Mn0.7O3—Zr0.89Sc0.1Ce0.01O2-x layerrespectively as cathode layers are shown in FIG. 6. Both of the tworeactors show much improved NO conversion rate after stopping the 50 mAof electrical current than before applying the electrical current. Thehigh NO conversion rate after stopping the electrical current might berelated to the reduction of Ni species in NiO and Ni and Mn species inLa0.6Sr0.4Ni0.3Mn0.7O3 while applying the electrical current, whichcould generate oxygen vacancies. These vacancies are potential activecenters for the adsorption and further decomposition of NOx. Thisexperiment further indicates that nitrogen oxide can be decomposed at ahigh conversion rate with less power consumption in a pulse mode ofapplying and stopping electrical current alternately than continuouslyapplying an electrical current.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

What is claimed is:
 1. A method for decomposing nitrogen oxide,comprising: contacting a gas stream comprising nitrogen oxide with adevice, the device comprising: a first electrode, an opposite secondelectrode, an electrolyte between the first and the second electrodes,and a power supply; and applying in a pulse mode an electrical currentfrom the power supply to the first and the second electrodes todecompose nitrogen oxide.
 2. The method of claim 1, wherein the step ofapplying the electrical current is at a temperature in a range of from300° C. to 1000° C.
 3. The method of claim 1, wherein in the pulse modethe electrical current is applied for a time period different from atime period when the electrical current is stopped.
 4. The method ofclaim 1, wherein in the pulse mode the electrical current is applied fora time period the same as a time period when the electrical current isstopped.
 5. The method of claim 1, wherein the electrical current isdirect current.
 6. The method of claim 1, wherein the first electrode isan anode.
 7. The method of claim 1, wherein the first electrodecomprises a material for oxidizing oxygen ions to oxygen.
 8. The methodof claim 1, wherein the second electrode is a cathode.
 9. The method ofclaim 1, wherein the second electrode comprises a material fordecomposing nitrogen oxide.
 10. The method of claim 1, wherein thedevice comprises an adsorption layer disposed over the second electrode.11. The method of claim 1, wherein the second electrode comprises anadsorption material for adsorbing nitrogen oxide.
 12. The method ofclaim 1, wherein the apparatus is of a tubular configuration or a planarconfiguration.
 13. The method of claim 1, wherein the device comprises acurrent collector.
 14. An apparatus for decomposing nitrogen oxide,comprising: a gas source for providing a gas stream comprising nitrogenoxide; and a device in fluid communication with the gas source andcomprising: a first electrode, an opposite second electrode, anelectrolyte between the first and the second electrodes, and a powersupply comprising a controller for applying in a pulse mode anelectrical current from the power supply to the first and the secondelectrodes to decompose nitrogen oxide.
 15. The apparatus of claim 14,wherein the first electrode is an anode.
 16. The apparatus of claim 14,wherein the second electrode is a cathode.
 17. The apparatus of claim14, wherein the second electrode comprises an adsorption material foradsorbing nitrogen oxide.
 18. The apparatus of claim 14, wherein thedevice comprises an adsorption layer disposed over the second electrode.19. The apparatus of claim 14, wherein the device comprises a currentcollector.
 20. The apparatus of claim 14, wherein the gas source is anexhaust gas source.